The present disclosure relates to a power reserve apparatus, a power system, and an electric vehicle that utilizes power from the power reserve apparatus.
In these years, the usage of a secondary battery such as a lithium-ion battery is rapidly expanding to a power storage apparatus for reserving power and a power storage battery for a car combined with a new energy system such as a solar cell or wind power generation, and the like. When a large number of power storage elements such as, for example, unit batteries (also called electric batteries or cells; in the following description, referred to as battery cells as necessary) are used to generate high power, a configuration in which a plurality of power storage modules are connected in series with one another is adopted. In the power storage modules, a plurality of, that is, for example, four, battery cells are connected parallel to, and/or in series with, one another to configure a battery block. A large number of battery blocks are stored in a protection case to configure a power storage module (also called an assembled battery).
Furthermore, a battery system is known in which a plurality of power storage modules are connected to one another and a common control apparatus is provided for the plurality of power storage modules. In this configuration, each power storage module includes a module controller, and the module controller and the common control apparatus communicate with each other through communication means.
In the case of using a plurality of battery cells, even when one of the plurality of battery cells has reached a lower limit of working voltage during discharging, other battery cells may not have still reached the lower limit of the working voltage due to differences in self-discharge between the battery cells or the like. When the battery cells are charged again in such a state, some battery cells cannot be fully charged, thereby posing a problem in that the capacity of these battery cells cannot be fully utilized.
In order to correct variation in voltage between the plurality of battery cells, an equalization process for controlling the balance between the battery cells is performed. Because a power reserve apparatus includes an extremely large number of battery cells in the case of an on-vehicle battery, a domestic power storage apparatus, or the like, there has been a problem in that it takes a long time to equalize the voltages of all the battery cells.
By intra-module equalization (inter-cell voltage adjustment), in which a process for equalizing the voltages of the battery cells is performed for each power storage module, and inter-module equalization (inter-module voltage adjustment), in which a process for equalizing the voltages of the power storage modules is performed, it is possible to reduce the time taken to complete the equalization.
As a configuration for equalizing the voltages of a plurality of battery cells in a power storage module in which the plurality of battery cells are connected to one another, a voltage balance circuit (referred to as the passive balance adjustment circuit) is known in which a resistor and a switching element are connected parallel to each cell and that causes a battery cell whose voltage is high to discharge. Furthermore, in a power reserve apparatus in which a plurality of power storage modules are connected to one another, a configuration is known in which, as with the above example, a resistor element and a switching element are used to equalize the voltages of the power storage modules.
However, when voltage is equalized by resistors, the voltages of the battery cells and the power storage modules undesirably decrease as a whole, and therefore a large number of times of charging and a large number of times of discharging for adjusting the voltages need to be repeated in order to set these voltages to a target value. Therefore, it takes a long time to complete the voltage adjustment of the power reserve apparatus. Especially when there is cyclic deterioration of a positive/negative active material, variation in the temperature condition, or a decrease in the amount of power in a battery cell according to an increase in internal resistance, the balance between the amounts of power in the battery cells is lost to a great extent and it takes a longer time to complete the voltage adjustment. Furthermore, the variation of the battery cells cannot be adjusted between the modules, and it is difficult to keep the balance.
On the other hand, a circuit (referred to as an active balance adjustment circuit) has been proposed that equalizes voltage using coils and switching elements. In a circuit configuration that equalizes voltage by moving electric charge using coils, voltage can be equalized in a relatively short period of time, but the arrangement of the coils and the switching elements and the control of the switching elements are complex, and a large number of coils need to be arranged in such a way as to enable smooth transmission of power.
For example, in Patent Literature 1, a large number of battery cells are divided into a plurality of series cell groups, while providing an inter-cell voltage balance correction circuit for each cell group and an inter-group voltage balance correction circuit. The inter-cell balance correction circuit equalizes the voltages of cells in each cell group using inductors and switching elements. The inter-group voltage balance correction circuit has a configuration in which the balance between the series voltages of the cell groups is corrected by alternating-current coupling formed by transformer coils and switching circuits.